The invention relates to a glass or glass-ceramic composition especially suitable for sealing to aluminum-based alloys.
In applications requiring the passing of electrical leads through an insulator wall such as in a vaccum tube or an explosive detonator, there is a need for an insulator which readily forms complex mechanical seals, has low gas permeability, has high chemical durability, has high mechanical strength, has high electrical resistivity, and has a thermal expansivity close to that of the metal to be sealed.
In the past, glass-ceramic compositions have been devised for sealing to molybdenum (U.S. Pat. No. 3,957,496) and to stainless steel (U.S. Pat. No. 4,135,936). These compositions require temperatures in excess of 900.degree. C. to form the seal and to devitrify the glass. Designers have generally avoided using aluminum in glass-to-metal seals because of the lack of glass composition with the desired thermal coefficient of expansion (235.times.10.sup.-7 .degree. C..sup.-1) which would form a seal to aluminum below its melting point (660.degree. C.).